1. Field of the Invention
The present disclosure generally relates to image forming and, more particularly, to an image forming apparatus having a function for correcting relative positions of a plurality of images in different colors (i.e., image registration correction control function).
2. Description of the Related Art
An electro-photographic type image forming apparatus forms a latent image on a photosensitive drum by exposing the photosensitive drum to light beams emitted from an optical scanning unit, and visualizes that latent image as a toner image by developing the latent image with toner. The toner image formed on the photosensitive drum is transferred onto a recording sheet or an intermediate transfer belt, fixed to the recording sheet by heat or pressure from a fixing unit, so as to be output as an output document.
Generally, the optical scanning unit includes a light source, a rotational polygon mirror rotationally driven to deflect light beams emitted from the light source, and a lens and a mirror for guiding each of the light beams deflected by the rotational polygon mirror to a photosensitive drum. These components are contained and held within an optical box.
In recent years, a configuration including a single optical scanning unit for exposing photosensitive drums of a plurality of colors has been becoming a major configuration of a color copying machine rather than a configuration including a plurality of optical scanning units corresponding to respective colors. In the above-described configuration, in particular, the optical box of the optical scanning unit is thermally expanded when an interior temperature of the image forming apparatus is changed. In addition, due to the influence of thermal distribution generated within the optical box, complex thermal expansion and deformation is caused within the optical box because of the anisotropic characteristics of glass fillers included in the materials of the optical box, so that the positions of the lens and the mirrors are changed. Therefore, a position of the light beam guided to the photosensitive drum is changed, so that an image cannot be formed on a target position if image forming processing is executed in such a condition. For example, in the image forming apparatus for forming a full-color image by superimposing images in respective color components of yellow, cyan, magenta, and black on a recording material, tint of the full-color image is changed because relative positions of the images in respective color components are misregistrated.
In order to solve the above problem, image registration correction control (color misregistration correction) has been known. For example, relative positions of measurement pattern images in respective color components are calculated by forming the measurement pattern images on the intermediate transfer belt at a predetermined timing and reading the measurement pattern images by a sensor, and writing timing of the light beams for forming images in respective color components is corrected. Through this processing, the images in respective color components are formed on desired positions, and thus the color of the full-color image can be controlled to have a desired color. However, in the color misregistration correction, because a correction pattern has to be formed at an appropriate time interval, number of printing sheets, and a temperature change amount, downtime is caused in the image forming apparatus, so that the productivity is lowered.
Japanese Patent Application Laid-Open No. 2012-42752 discusses an image forming apparatus which executes a first image registration correction control for actually measuring positions of measurement pattern images of all color components and a second image registration correction control for estimating positions of the measurement pattern images of the color components other than two color components based on a measurement result of the positions of the measurement pattern images of the two color components. The image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2012-42752 executes the first image registration correction control before starting image formation in a case where a print job is input thereto after more than two hours has passed from the end of the previous printing operation while a difference between the temperature of the fixing unit and the ambient temperature is greater than 5° C., while executing the second image registration correction control in other cases.
Because the second image registration correction control is executed based on a predetermined condition, the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2012-42752 can reduce the downtime compared with the case where the first image registration correction control is executed consistently.
In the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2012-42752, the downtime is caused in a period before resuming the printing operation because both the first image registration correction control and the second image registration correction control are executed before starting the image forming processing (printing operation). Therefore, productivity of the image forming apparatus is lowered.
In the second image registration correction control, relative positions of the images in all of the color components cannot be corrected with high accuracy because positions of the images in color components other than a reference color and a target color are estimated.